This invention relates to a vehicle navigation system, and more particularly, to a vehicle navigation system having a function to calculate a travel time to a destination.
Such a navigation system has two main and interrelated functions: the first function is to display to the user a map of the present vehicle location; and the second function is to provide guidance on a guided route from a starting point to a destination. The second function is generally referred to as route guidance or selection. A route is typically optimized for one of various parameters, for instance, minimum travel time, minimum travel distance, maximum use of freeway, or other parameters pertaining to movement of a vehicle on a road network.
Another function of such a navigation system relating to the above two main functions is a function to calculate and display a travel time to a destination. Usually, a navigation system has map data and searches the data to determine an optimum route. The map data include travel times of all the road segments in an area encompassing a starting point and a destination point. These travel times are obtained, for example, by actually driving those road segments under various road conditions and averaging the actual travel times. The travel times may also be obtained, without actually traveling the road segments, by predicting travel speeds based on the road characteristics of the segments, such as a road width, road type, etc. The predicted travel speeds may be adjusted according to posted speed limits.
Conventionally, a travel time to a destination is calculated by adding these travel times in the map data along the road segments that are yet to be traveled along the optimum route. The problem with this conventional method of estimating a travel time to a destination is that the estimated travel time necessarily includes an error because the travel times in the map data do not accurately reflect the particular driving conditions to which the vehicle is actually subjected.